Most people probably do not question the fact that human blood stem cells – those that give rise to all the cells in our blood – live inside the marrow of our bones, called a stem cell “niche”. But it is pretty odd when you stop to think about it. I mean, it makes sense that the hard, calcium-rich structure of bones provide our bodies with a skeleton but why is it also responsible for making our blood?

This week, researchers at Harvard report in Nature that the answer may come down to protecting these precious cells from the DNA-damaging effects of UV radiation from the sun. They arrived at those insights by examining zebrafish which harbor blood stem cells, not in their bones, but in their kidneys. Fredrich Kapp, MD, the first author of the report, was trying to analyze blood stem cells in zebrafish under the microscope but noticed a layer of other cells on top of the kidney was obscuring his view.

In a zebrafish larva (illustration above), a dark umbrella formed by pigmented cells (white arrows point to these black spots in box, left) in the kidney protects vulnerable stem cells from damaging UV light. Right image is a closeup of the box. Scale bars equal 100 micrometers (left) and 50 micrometers (right). Credit: F. Kapp et al./Nature 2018 Read more at: https://phys.org/news/2018-06-blood-cells-bones.html#jCp

That layer of cells turned out to be melanocytes which produce melanin a pigment that gives our skin color. Melanin also protects our skin cells from the sun’s UV radiation which damages our DNA and can cause genetic mutations. In a press release, Kapp recalled his moment of insight:

“The shape of the melanocytes above the kidney reminded me of a parasol, so I thought, do they provide UV protection to blood stem cells?”

To answer his question, he and his colleagues compared the effects of UV radiation on normal zebrafish versus mutant zebrafish lacking the layer of melanocytes. Confirming Kapp’s hypothesis, the fish missing the melanocyte layer had fewer blood stem cells. Simply turning the normal fish upside down and exposing them to the UV rays also depleted the blood stem cells.

And here’s where the story gets really cool. In studying frogs – animals closer to us on the evolutionary tree – they found that as the tadpole begins to grow legs, their blood stem cells migrate from the melanocyte-covered kidney cells to inside the bone marrow, an even better form of UV protection. Senior author Leonard Zon explained the importance of this finding:

“We now have evidence that sunlight is an evolutionary driver of the blood stem cell niche. As a hematologist and oncologist, I treat patients with blood diseases and cancers. Once we understand the niche better, we can make blood stem cell transplants much safer.”